Such a phosphor is disclosed in U.S. Pat. No. 4,423,349 to Nakajima et al. In that patent, phosphor brightness is optimized by varying the proportions of lanthanum, cerium and terbium. The brightest phosphor has the composition La.sub.0.2 Ce.sub.0.7 Tb.sub.0.1 PO.sub.4 and is made by a multiple-step synthesis.